High-level recording system



July 15, 1952 E. G. cooK HIGH-LEVEL RECORDING SYSTEM 4 Sheets-Sheet 2 Filed Feb. 26, 1948 INVENTOR EMORY G. COOK ATTORNEY July 15, 1952 E. G. COOK HIGH-LEVEL RECORDING SYSTEM 4 Shees-Sheet 3 Filed Feb. 26, 1948 .om @E INVENTOR EMORY G. COOK RN. i

ATTO

July 15, 1952 E, G, COOK 2,603,720

HIGH-LEVEL RECORDING SYSTEM Filed Feb. 26, 1948 4 Sheets-Sheet i AMPLIFIER AMPLIFIER fw g f L 532 51M 562 I 526 f54 n Jv B-o 562 "56 @h 556 J f 5&9 572 l J f ll Ihmcntor IIVIORY G. COOK records, especially of music.

Patented July 15,

UNITED STATES PATENT OFFICE t 'f f 2,603,720 l, A

molli-LEVEL RECORDING SYSTEM v n yEmoryd. oook, Floral Park, N. Y.

Y *Appiication' February 20, 194s, serial No. 11,242

Thisv invention relates to the recording of sound. More particularly, it relates to methods and apparatusv for producingimproved phonograph When sound is recorded for thel purpose of `makingphonograph records a'contin'uous spiral groove is formed on the surface of the record and is' modulated (caused to change direction) in accordance with the recorded sound signals. In other Words, the groove deviates laterally from a true spiral so that it traces a path corresponding in shape to the waveform of the rsound signals.

If these excursions or deviations of the groove, from the normal direction which the groove would have if it were not modulated, are not sufliciently large, the random noise effects, for example, as evidenced by needle scratch, which are an inherent part of any recording or reproducing,v

system, are large enoughcompar'ed'with the amplitude of the sound signals which can be repro-Y duced from the record, that the sound may be of poor quality. As the average amplitude of the deviationsY is increased, the sound or volume level on the record increases and the quality andutility of the recordingl increases correspondingly. -v

However,'the amountby which thev amplitude or sound levelV on the record may be increased is limited by several factors.Y For example, the maximum swing of the stylus whichy cuts the rec-v ord must ,be less than one-half the distance between the centers of adjacent grooves',so that a dividing wall remains between the grooves to guide the reproducing stylus and prevent it from jumping from one groove to'another. Furthermore, these walls must have sufficient horizontal thickness to withstand the lateral forces exerted by the reproducing stylus so that the wallswill not break or chip'during-'repeated playing of the record. r

It is possible to increase the average amplitude of the groove by the use'r o'f a limiter circuit, which automaticallyreduces amplitudes of all of the frequency components of the signal whenever the maximum amplitude ofthe signal being recorded exceeds a predetermined level. The operating level of this limiter is adjusted so that a suitable dividing wall is insured between adjacent grooves.- Al l Howeventhe characteristics of conventional recording'systems are such that low' frequency signals are 'recorded'at higher relative amplitudes than higher frequency signals and, in accordance with one aspect of the present invention, excursions .ofthe recording stylus of excessive magnitude vare prevented-icy limiting only .the ampli Claims. (Cl. 179--100.1)

ltude of the low frequency components of the signal to a predetermined value. lIn this manner the selective limiting action is effective' only-on the low frequency components of the signal, and other frequency' components are recorded at fullvolurne level.

Accordingly, it is an object of this invention to provide a system of recording wherein the low'frequency components of thev sound signal are limited selectively to a predetermined maximum amplitude. y Y

Another limitation upon the level of sound which may be recorded isset by the angle of deviation of the groove, defined for the purpose of this description as the angle between the actual direction of the modulated groove in its lateral excursions and the direction which the groove would have, if it were-unmodulated, that is, if no sound signals were recorded.

When sound is being reproduced from a phonograph record, the record is rotated, usually at constant angular speed, by a motor or other drive means, while the stylus of the reproducing head merely rests upon the surface of the-record, As the record rotates, the stylus of the recorder head moves laterally in accordance with thev deviations of the groove as it traces the spiral from the outside tothe center of the record. Thus, it is the rotational force of the record which is translated, by the groove deviations, into the lateral forces on the reproducing stylus. As the angle of deviation of the' record groove increases, the forward component of the force exerted by the surface of the record upon'the reproducing stylus increases. At some critical value, this force becomes so large that the stylus will no longer follow the groove and will either chip the record orbe forced out of v the record groove. In practice, if the angle of deviation of thegroove is greater than 30 to 45 degrees, the reproducing' stylus will be unable to follow the sound groove.v 'l

Assume (however, only for the purposev of explanation) that a groove is modulated in accordance with a square wave as indicated schematically in Fig'. 1.` The direction which the groove would have if it were not modulatedv is shown by the line G-G. `The modulated groovefollows the contour indicated by the ,broken line 2. The maximum angle of deviation 0 between the modulated groove and the path which the groove would have if it were unmodulated is degrees. It is apparent that a conventional reproducing stylus will ,not follow the square waveform of the groove or' elsewill chip the'groove walls; This 3 would be true even though the square waveform of the groove 2 were a faithful reproduction of the electrical signals which were applied to the recording equipment. It follows, therefore, that a recording system which produces a sound groove corresponding most faithfully to the original sound signals, does not result necessarily, when considered in combination with the reproducing equipment, in the most desirable over-all system.

In a recording system the record blank is rotated in the 'same manner as in reproducing equipment, but the power which produces the lateral modulation movement of the stylus is provided by forcesY applied directly to the stylus and, although it is not possible with conventional recording equipment for the stylus 'to cut a groove having an angle of deviation'as high as ninety degrees, it is easily possible to cut a groove having such a high angle of deviation that the it. f

-j The elimination of high angleY deviations, in accordance with this aspect of the invention, enables the reproducingl stylus to follow faithfully jthe contour of the sound groove, thereby eliminating one source of distortion and decreasing the likelihood of chipping or wearing away of the walls which divide adjacent sound grooves.

. A`s the frequency of the recorded signal is increased `(w ith,-for example, the amplitude of the signal and the linear speed of the groove remaining constant) the angle of deviation of the Vgroove increases. Thus, it is the high frequency components of the recorded signals which produce the greatest anglesof deviation of the groove. Y

With a given frequency, the lateral velocity of the cutting stylus, and, therefore, theangle of deviation of the groove, depends upon the amplitude of the vsignal being recorded. This may be seen incre clearly by examining the modulated grooves represented in Fig. 2, where line G--Gagain represents thedirection of an unmodulated groove. A groove modulated with a triangular waveform, `with the height A, follows the path indicated bythe broken line 4, and the angle of deviation is indicated at p. If, however, the amplitude of this triangular waveform is increased from A to-B, the frequency of the recording signal and the linear groove speed of the record remaining constant, the groove will follow the path indicated by the solid line 6. It is observed readily; that the lateral velocity of the recording stylus must have been increased in order to cut the latter groove, and that the resulting angle of deviation has increased accordingly,k -as indicated by angle a. Thus, the angle of deviation may be controlled by limiting selectively Vthe high frequency components of the `signal while permitting other frequency components to be recorded at full volume level.

y Accordingly, it is a further object of this invention to provide a system for the high level recording of sound wherein the high frequency components of the signals are limited selectively, so that the maximum angle of deviation of the record groove does not exceed a predetermined value.

The angle of deviation of the record groove is a function also of the linear speed of the record groove at the point of contact with the stylus. In conventional sound recording and reproducing systems, the rotational speed of the record is held constant with the result that the reproducing stylus will be unable to follow 4. linear groove speed is higher at the outer portions of the record and decreases steadily toward the center of the record. The effect of this changing groove speed may be seen clearly by consideration of Fig. 3, in which the direction of the unmodulated groove is indicated again by line G-G. If a triangular waveform of a given frequency is recorded near the outer edge of the record where the linearvspeed of the groove is highest, Athe v'lateral movement of the"'recording stylus will cause a groove 8, having an angle of deviation, it, to be recorded on the record surface. If, however, an identical signal is recorded near the center of the record, say at a point where the linear groove speed has decreased to one-half its former value, the lateral movements of the recording stylus remain the same, but the groove formed by the stylus in the surface of the record follows the contour indicated by the broken line I2, having an angle o f deviation ,6, which is greater than angle gb, eventhoughjthe frequencyof the recorded signalhas remained constant.' Thus, in accord.- ance with still anotheraspect -cf-the present invention, further increase in .the'volume level'of the recorded sound isobtained by controllingEthe selective limitation ofV the ihigh frequency components in accordance with ythe linear speed of the record surface at the point" of contact with the stylus. Y Y

Accordingly, it is a further Yobject of this invention to provide a'system for the recording of sound vsignals wherein the amplitude of the high frequency components of the signal to be recorded are limited selectively'with the level of limiting action controlled'in accordance with the linear groove speed of the record at the point of contact of the stylus(V By combining both -of ltheselectiveV hunting actions, that is, by limiting boththe high and low frequencies selectively, the volume level which it is possible to place on the recordis increased by a surprisingly large amount. Actual records produced in accordance` with' the principles of the present inventioncarried a level of sound volume asfmuch as 12 decibels above records produced with conventional sound recording apparatus. In other words, ythese new records contain some 32 times as much lsound venergy and produce a sound many times higher in volume from the loudspeaker of the playback equipment.

Thus, it is another object of this invention to provide methods and .apparatus for recording sound signals Vat a high volume level wherein the high frequency andlowH vfrequency components of the sound signals :aremomentarily limited as necessaryselectively vinamplitude.

With such recording-the noise level, which may be noted upon reproduction as a background crackling or hiss, is reducedappreciably because of the lower amplification which is required in the reproducing equipment. As a result of this lowerA requirement in the way of amplification, a vcorresponding Yimprovement in the quality of reproduction-may Vbe achieved readily. j f Y When two signals of different frequencies, particularly a low frequency and a high frequency. are passed simultaneously through a nonlinear device, for example, an amplifier tube operating outside its linear range, newfrequencies are produced in addition. tothose already present. If such action occurs in either the recording or reproducing'equipmentythe sound waves radiated by the loudspeaker will contain frequencies that were not -present inthe original sounds. This is known as intermodulation distortion and occurs most noticeably; in the usual amplifier arrangements, when high -an'lplitude signals are present because there is vmore likelihood that these signals will cause the amplifier tubes to operate on nonlinear portions of their characteristic curves.

VIn laccordance with the present invention the extent of' such-intermodulation is reducedinboth 'therecording and reproducing equipment.

recording system and having wide utility in other elds'of application. For' example, it is an object of this invention to provide apparatus for producing an impedance to the ow of alternating current, which impedance may be varied linearlyover Wide ranges by the application of low direct control voltages.

Accordingly, the objects of this `invention are directed to methods and apparatus for use in the production of improvedy sound records and to the process of frequency selective amplitude limitation of sound signals whereby phonograph records may A be produced having a higher sound volume level without increasing the necessary groove width. q.

Other objects and advantages will be in part pointed out and inpart apparent from a consideration of the following description taken in conjunctionwith the accompanying drawings, in which: f y Fig. l shows a groove modulated with a square wave and illustrates the importance of the angle of deviation of the groove in the process of reprod'ucing recorded sound signals;

Fig. 2 is a diagrammatic representation of the paths of two grooves and illustrates the effect of amplitude of sound signals on the angle of deviation of the record groove;

Fig. 3 isa diagrammatic representation of the paths of two' grooves illustrating the effect of linear groove speed on the angle of deviation of the modulated groove;

Fig. 4 is a block diagram illustrating a record-y ing system utilizing selective limitation'v of the sound signals;

Fig. 5 is a block diagram illustrating a recording system'incorporating separate selective limitation of the'highand low frequencies;

Figs. 6 and 6A'showa detailed circuit diagram of a recording system control constructed in accordance with the present invention;

Fig. 'l shows alternative apparatus for selective limitation of the sound signals; and

Fig'. 8 shows'the circuit diagram of a variable impedance generator suitable for use in the apparatus of Fig. '7.

The control of the signal which is to be recorded may be accomplished by various arrangements of electronic apparatus and the following examples are given for the purpose of illustrating the best embodiment now known, but are y6 lwhich may include'various mixing and auxiliary'- control circuits such as are employed in the 'usuali recording system. The output of amplifier 16, is coupled to" -a frequency selective limiter I8. The limiter i8 may be onedesigned toV limit only the amplitude of the high frequencies to avoid excessive angles of deviation,'or it may be one adapted to limitonly the low frequencies in order to prevent excessive excursions-of the record groove from its normal path. `In either case, the medium frequencies, that is, those frequency componentsin the middlerange between the high and low frequencies, are permitted to pass through this portion of the recording equipment without any limiting action'. Theoutput signals from the limiter I3 are applied tcaconventional amplierfZZ, the output of which Yiscoupled to a recorder'head 2li'.

In order tolimit both thehigh kand low frequency components selectively, an arrangement may be employed such asis shown in block diagram form in'Fig. 5. The sound signals which are to be recorded are translated by a microphone |4A into electrical signals.y For convenience; the paths of the high, low, and medium frequency components of theseelectrical signals are represented respectively, by adotted line 26, a wavy line 28,' and a dot-dash line 32. These signals are amplified by amplifier IBA in the usual manner and applied to a dividing network 34 which separatesthe composite signal into the three components, that is, high, low, and medium frequencies, as indicated by the 'separation of lines 2t, 28, and 32l from the dividing network 34.

The frequencies lying in the middle frequency range are amplified-by a'conventional amplier circuit 36 in which nolimiting action occurs.

The high frequency components represented by line'` 26 are applied to Va limiter-amplifier 38, Where these frequencies are amplified and limited, that is, prevented'from-exceeding apredetermined amplitude. In-other words, limiter-amplifier 3B is arranged so that as the high frequency signals approach a predetermined amplitude, the gain of amplitude 38 is reduced automaticallyfto prevent the signals which are delivered by limiter-amplifier. 38 from exceeding, at anytime, this predetermined value. Such a limiter-amplifier may be controlled by a suitable control voltage applied .to a lead 42, so that the predetermined voltage at which the limiting action occurs ,maybe varied in accordance with other parameters. thelevel at which thel limiting action takes place may be controlled, in accordance `with the linear A velocity of the record at the point of contact with the stylus, by applying to lead 42 a control Avoltagewhich is a function of the lateral position of the recording head 24A on the record.

The low frequency components,l indicated by wavy line 28, are applied to another limiter-amplifier 44, the operating level of which may be controlled by a voltage applied to lead 46.l

The limiters 42 and 45 and amplifier 36 are arranged so that the signals delivered by the respectve equipments are in proper phase relationship, so that they may be recombined in amplifier 22A and subsequently applied to the recording head 24A.

i Circuit details Figs.' 6 :and `6A together illustrate the details of one form of apparatus for recording sound signals in accordance with the present invention. The electrical signals corresponding to the sound signals which are to be recorded are delivered by Thus, as pointed out above,

7 a'inicrophoneivMB to a preamplifier, indicated in blockf-form at 48H. yThe output Vsignals from the amplifier 48- are -coupled by means of a transformer 52 which has a secondary winding 54 to a potentiometer -56,.the variable tap 58 of which is connected to a controlgril 620i a pentode vacuum tube 64h; Oneend of the potentiometer 56 isconnectedto'ground by means of alead 66 so that the level of signal applied to control grid 62 maybe varied by adjustment of the poten- Ytiometer 56.A i n y Separationof high frequency components The suppressor grid 68'of tube 64 is connected to cathode 12 which is connected to ground through a'bias resistor '.14 infthe conventional manner. Voltagefis provided'for screen grid 16 throughla lead 18 which 'is connected to a suitably regulated power source A (not shown). Anode voltage for tubev 64 is supplied to anode 82 through load resistors 84 and 86 from a lead 88 which is connected to a suitably regulated power source B (not shown). A condenser 92 and a series resistor 94 are connected between anode 82 and ground. A potentiometer 96, which desirably has a resistance value that is high relative to the value of resistor 94, is connected in shunt lwith resistor 94. Resistors 86 and 94 preferably have substantially equal values. y n v This Varrangement provides a simple dividing network which separates the high frequency components from the low and middle frequency components. The alternating current load impedance in this circuit is small as compared with the very high plateresistance of the tube 64 :and the tube, thus, may be considered to act as a constant currentr generator, which witha given signal voltage applied to control grid 62 will cause the same amount of current, irrespective of Ifrequency, to flow through the plate loading network. The condenser 92 has a relatively low reactance compared ,totheresistance of resistor 84 at thehigher frequencies, and a larger reactance as the frequency is decreased. The valuesof resistors 86 and 94 are low, relative 'tothe value of resistance 84. The

signal current from anode 82 will divide, thus, with the higher frequency components passing through condenser 92 and resistance 94 to'ground andthe low and medium frequency components passing through resistors 84 and 86. TheV output signal voltage at point 98, that is, the voltage developed acrossY resistor 86, will comprise the low and' medium frequencies and will be complementary to the high vfrequency voltage developed at point |02, that is,the vvoltage developed across resistor 94. In other words, with a given signal voltage applied to grid 62, the sum of the voltages at points 98 and |02 will remain constant, regardless of frequency. Y

The low and medium frequency components indicated, respectively, by the .wavy and dot-dash lines adjacent lead |04, are connected to the circuits shown in Fig. 6A, which will be described later.

High frequency" limiter-amplifier 'Y The high frequency components, represented by the dotted line adjacent lead |06, are applied, from an adjustable tap |08 of potentiometer 96, to control grid ||2 of a limiter-amplifier tube I4. The suppressor grid I6 of this tube is connected to the cathode ||8 which is in turn 'connected to ground through a bias resistor- |22. Grids |24 and |26 of tube I4 are connected by lead |28 to powersource A; ,The anode ,|32of this tube is 8 connectedthrougha load resistance |34 to power source B. ,The signal yoltage whichuis developed across load resistor |34 is c'z'ouple'd through a condenserv |36, anisolatingresistorV |38, and a con- .denser |42 to 'anampliiien indicated in block form atl44.` i.

IThe` amplifcati'onof )limiter-tube |4 isa function ofthe bias or control voltage which is applied to control grid |46 of that tube. This control voltage limits the amplitude ofthe high frequency components and is Varranged to control this action in accordance Iwith' the -high frequency amplitude'of the composite signal and with the linear groove speed of the record at the point of contact with the stylus, as described below. I l v- The low and medium frequency components are returned from-the equipment represented by Fig. 6A; by a lead |48, to .point |52 'where they are recombined withY the high frequency-components from limiter-,amplifier |4 and applied to the .amplifier |44. Y

The joutput signal from amplifier |44 is applied through leads |54 and |56 to a magnetic recorder head |58 which is moved laterallyacross turntable 62 by rotation of a feed screw |64.

Control 'circuits for high frequency limiteramplifier.

The instantaneous velocity of the stylus will be proportional to the signal voltage on leads |54 and |56. Accordingly, the instantaneous arnplitude of this signal voltage is a measure of the angle Yof. deviation of the record groove. This voltage isutilized to control the limiting action of tube.||4, Y 'y l' The primary|66 of a transformer |68 is connected across leads |54 and |56 by leads |12l and |14. A potentiometer |16 is connected across secondary winding |18 of transformer |68 and one terminal thereof is connected to ground by means of arlead |82. The variable tap |84 of potentiometer |16 is connected to ground through a potentiometer generally indicated at 86. This potentiometer may comprise a continually variable resistance potentiometer, which'is provided with a tapered characteristic either by mechani- V204, shaft 206, worrn'l208and gear 2|2, to. rotate rotor arm |98.

The .voltage vbetween'rotor arm |98 and ground is utilized toicontrol the limiting action of tube ||4. ThisA voltage whichV is directly proportional tothe lateral Avelocity of the cutting stylus, is derived from potentiometer |16 and is attenuated by potentiometer |86 in accordance with the lat- -eral position vof recording head |58. Thus,v when recording head |53 is at the outermost portion of the record, the limiting actiontakes place at a higher signal level because of the relatively high lineargroove speed of the record and rotor arm |98 is positioned-accordingly, in the lower portion ofpotentiometer |86. f As the recorder head |58 moves toward the center of the record, rotor vamr-|98 progresses in a counterclockwise direction toward resistor |88, thus lowering the signal Ilevel-at which limitingzaction; takes place as recorder head |58 approaches theycenter of the record. Y

Rotor arm |98 isconrnected by a lead 214 'to a grid 2|8 of a triode tube 222, the 'cathode 224 of which is connected to ground through a resistor 226 which is shunted by a 'capacitor 228. The anode 232 of this tube iscoupled through a condenser 234 and a resistor 236 to lead 238 which is connected to the kcircuits shown in Fig. `6A to' provide a volume indication as described below. Anode 232 is connected vthrough plateY load resistance 242 to power source-B.` The signal voltage which is developed 'across load resistance 242 is coupled through a condenser' 244 to a cathode 246 of a diodetube 248. This cathode is biased positively with respect to groundby a voltage divider circuit comprising a resistor 252 and a potentiometer 254 connected between power supply B' and ground. The variable tap 255 of potentiometer 254 is connected through an isolating resistance 258 to the cathode 245. Anode 262 of diode 248 is connected through a load resistance 264 to ground, With this arrangement signal current will ilow through diode 248 only when the negative amplitude of the signal voltage applied to cathode 246 through condenser 244 exceeds the positive bias applied to the cathode by means of potentiometer 254, the point at which current starts to flow; and accordingly the level at which limiting action takes place is adjusted by varying the position of variable tap 255 of potentiometer 254.

The rectified signal voltage appearing across load resistance 264 is smoothed by a condenserresistance lter network comprising series resistance 266 and shunt capacitors 268 and 212. This circuit delivers a negative direct potential which is applied by lead 214 to: control grid |48 of tube Thus, negative biasis applied to grid |48 of tube I4 whenever the-voltage appearing between leads |54 and |56 exceeds a predetermined value, which value is a function of the velocity of the recording stylus andof thejlinear groove speed of the record at the point fof contact with the stylus. This negative bias on grid |46 reduces the amplification of tube ||4,- thus lowering the amplitude of the high frequency signals which are applied to amplifier |44.

Separation of low from medium frequency Y components to ground through a bias resistor-292. The screen grid 294 is connected t9. power source A. The anode 296 of this tube is connected to a dividing network similar. to thatdescribed in connection with tube`64'in Fig". '8, except that in thiscase the constants are adjusted lso'th'at the low and 'medium frequencies v are separatedy from each other.

Th first prises series plate load resistors 2 88an'd 302 *which are connected to power source' B; "The second branch of the network, comprises capacitor 304 in series with resistance 306 :which -is vconnected to ground asin the-nrstidividing network. The

branch of the'.dividing'inetwrk comresistance values of resistors 302 and 306 are desirably equal and the reactance of condenser 384 is substantiallyequal to thevalue of resistance 238 at *the crossover point of the network. characteristics.1

The mediumfrequency components are coupled from 'point 308 between resistor 308 and capacitor 304, to grid l3|2 of triode tube 3|4. The cathode 3|6 of this tube is connected toy ground in the conventional mannerv throughy a bias fresistor `318. Thelanode'322 of this 'tube is coupled through series load resistors/324 and'326 to power sourcef'B. The outp'i'it'sign'al is coupled from point 328 between these A"t'wo resistors, which thus form a voltage divider, through secondary winding332 of a transformer 334, where'the low frequency components are' reinsertedand'through a coupling condenser 7335 to a control grid 331 of a triode vacuum tubef339. l

Low frequency vemplirte-rlimiter VThe low vfrequency components ofjthe signal are taken from the plate circuit of tube `282 at point 340 and, Vby lead 34| Vcondenser 342, and potentiometer`343, coupled to grid 344 of an amplier-limiter tube 348. The suppressor grid 348 of this tube is ,connectedy tothe cathode 352 which is connected to ground througha ybias resistor 354.. Grids 356 and 358 of this tube are connected to thepower source A. The anode 382 of tube 346 is connected by lead 334 to the'primary 3680itransformer 334, the center, tap of Vwhich is connected to power source B.

The lcwffrequencysignalsare` amplified by tube 343 to an .extent depending upon the bias voltage whichgis applied to control grid 368. of that tube. This Voltage'is .controlledso as to limit the amplitude of the low-frequency ,components of the. signal whenever theiamplitude of the low andmedium frequencyl components of :the signal exceed a predetermined value..A

. kCfoutrol*f'zircuit for lou.) frequency amplifierr limiter The voltage developed by the low and medium frequencies at point 312 in the'plate circuit of Ytube 282, is applied, through leadr 314,-coupling condenseri316, and'potentiorneter 318 to control vgrid 382 'of triode tube'384; f The cathode 383 of this tube is 'connected to ground through bias resistor -388 in parallel with a condenser 382. Anode394 of tube 384 is connected through a load resistor 336 to power ysource B; and is coupled'also through a condenser 398 to cathode 402 of a biased diode tube 484. This cathode is biased positively with respect to ground by a voltage divider comprisingja'resistor 406 and a potentiometer 408 connected between power source B and ground. The adjustable tap 4|2 rof this potentiometer is connected throughl a resistor 414 to the cathode '402. The anode als or tube 404 isconnected to l'ground through a' load resistor 4|8. f

' Thus, icurrent now is permitted throughdiode 484 vwhenever the negative amplitude of thelow 'and medium frequency :signal components, de-

yelopedA across load resistor l336 of tube 384, exceeds the value of the positive-bias applied to cathode 482 from potentiometer 408.- yWhen this 'occurs' a direct voltage'is-developedacross'load resistor 4|8 causing anode 4|6'to`become negative with respect to ground. This direct Voltage is filtered by a resistor-condenser network compris'ing a series resistor 422 and shunt ,oondensers Y A ,Y Y .11 Y -424 and 426, and applied to control'grid V368 of limiter tube 346. 1

Thus, Whenever the-amplitude of the medium and low frequency components of the signal exceed the predetermined value which'would cause the recording stylus to exceed the'limits set by the. normal groove separation of the record, a negative. voltage Vis applied to grid 368 of the amplifier-limiter tube-3146; thus reducing the amplification of this tube; so that the low frequency components, which are' combined with the medium*V frequency components Yby transformer 334,are limited in amplitude and prevent thereby excessively -large Yexcursions of the modulated groove.'Y 1 "r V In orderxto' preventv transient effects which might otherwise be introduced by application of limiting control voltage to vgrid 368 of tube 346, this control voltage is applied simultaneously to grid 428 of`a tube 432 which has characteristics substantially identical With those of tube 346. Suppressorfgrid 434 -of'this tube is connected to cathode 436, which is inturn connected to ground through a bias ressistor 438. Grids 442 and 444, correspondingY to grids 356 and 358 in tube 346, are connected to power source A. Control grid v446 ofvtube 432, which corresponds to control grid 344 in tube 3146 to which the low frequency signal voltage is applied, is connected directly to ground... Anode 44-8 of tube 432 is connected by lead 452 to the opposite end-of primary winding 366from that which is connected to anode 362 of tube 346.

Recombiningof. low with medium frequencies Withthis arrangement thelow frequency sig nal components produce-a current inthe anode circuit of Vtube 346Whichflows through one-hali of the primary winding 356 of transformer 334, and thus induces corresponding signal voltages `in secondary winding 332. However, the direct limiter control voltage is applied to control grids of tubes 346 .and 43-2. Th-e plate current signals produced thereby flow. in opposite directions, through their respective halves of the primary winding 366V and thus cancel, so that no voltage is induced thereby in the secondary winding 3'32.

' The low frequency Vsignalstwhich .have znow been limited to a predetermined maximum amplitude are applied through condenser 335 to grid V331 of tube 339 simultaneously with the medium frequency signals from the anode circuitof tube 3| 4, thetubes, and windings of transformer 334, being arranged so that the W and medium frequency`V signals are recombined in proper phase relationship.

Recombim'ng of low and medium frequencies with Y high, frequencies `Anode462 of this tube is connected through resistor 464, lead |48 and (see Fig. 6) load resistor 466 to the power source B. The lowand medium nfrequency components are recombined at point |52 vwith the high frequency components of the 'signal to'forin the composite audioV signal.V Thus,

Aampliier |44'to'the recorder head |58. 1

` .Volume Vlevel .indicator In orderio provide a visual indication of u1 Vrecording level of the system. a meter 468 (Fig.

6A) is provided. A protective resistance-412 is connected across the terminalsY of the meter, one of which is connected to ground by alead 414. The other terminal of the meter is connected through a condenser 416, an isolating resistor 418, and lead 482V to the anode circuit of -tube 384 in the low and medium frequency limiter control circuit. The same terminal of the meter is connected also through lead 238 (see Fig.l 6), resistor 236, and condenser 234to the anode circuit oftube 222 in the high frequency limiter control circuit.`

Approximate values lof the components which may be used in the above circuits are given in the following table. v Y

' Tubes' Resistance elements value in thousands of ohms Value in thousands Value'n I Mcrofarads Value in Number Microfarads 99999999 O meses Although actual detailed circuits have been provided togetherwith typical values of the components, it is to be understood that this example is given, in accordance'with the'statutory requirements,V for the purpose of exemplifying a now preferred embodiment, and that the invention is not intendedvtobe limited thereby.'

F'or example, the selective. limiting action may be accomplished in the manner indicated in Fig. 7. The electrical signals are delivered by a microphone |4C toa conventional amplifier 562 indicated in block form;V Y The output from this ampliner is coupled to asecond amplifier 504 through V ponents are attenuated by voltage drop` across resistor 506, therefore, is a function of the impedance presented by impedance generator 5|4. Such an impedance generator, if controlled in accordance with the present invention, may be utilized to provide the selective limitation of the high frequency components.

In order to limit selectively the lower frequencies, yan inductance 5I8 is connected from point 5I6 to ground through an impedance generator 522, indicated in block diagram form. By controlling the value of the impedance presented by generator 522, in accordance with the principles of the present invention, the low frequency components of the signal may be limited selectively. f

Fig-8 is a circuit diagram of an impedance `generator suitable for use las vthe blocks 5|4 and 522 in the system shown in Fig. 1. AThe desired Alimiter control rvoltage is applied simultaneously to control grids 524 and 526 of tubes 528 and 532, respectively. by means of a lead 534.` The suppressor grids 536 and 538 of these tubes are connected to their respective cathodes' 542r and 544 which are returned to ground through acommon bias resistor 546. The grids 548 `and 552 of tube 528, and grids 554 and 556 of tube, 532, are connected to power source A. Control grid 558 of tube-528 and the corresponding control grid 562 of tube 532 are returned to ground respectively through grid return resistors 564 and 566. The anodes 568 and 512 of tubes 528 and 532, respectively, are connected to opposite ends of primary winding 514 of a transformer 516 the center tap of Which is connected to power source B. A resistor 518 is connected in shunt with one-half of Winding 514 and a resistor-582 is -in shunt with the other half of this. winding. Negative feedback voltage is applied from anode 568 to grid 558 of tube 528 by means of a condenser 584 and a series resistance 586. similar manner anode 512 of tube 532 is coupled to grid 562 through a condenser 588 and a resistor 582.

In operation, when an alternating current signal is applied to winding 594 of transformer 515,

a voltage is induced in winding 514 which is applied simultaneously, but with opposite polarity, through condensers 584 and 534 to grids 558 and v 562 of tubes 528 and 532 respectively.l The applied signal produces current fiow through these tubes and through windingy 51-4 of the transformer. This loading effect decreases the impedance of winding 594 to the alternating current signals. However, the amount by which this impedance is reduced depends yupon the .transconductances of tubes 528 and 532, which .have substantially identical characteristics. The transconductance of these tubes is a function of the control voltage applied to the control grids 524 and 526. The alternating current impedance presented by winding 594 may be varied accordingly by application of direct control voltage to lead 534. This arrangement provides wide variation in the impedance of y584 which is substantially resistive in character.

,114 It is obvious thatjthe structures for carrying out the present invention may vary widely from the illustrative embodiments presented herewith without departing from the sco-pe of the invention. For example, the signal which is utilized to control the limiting action of the low frequency limiter is derived from a particular` point in the circuit where low and medium frequencies are present only for convenience, and readily could be taken from some other point in the circuit.

y For example, this control signal could-be derived directly from the input signal from the microphone after suitable amplification. Other circuit modiiications are within the province .of one skilled inthe art who may modify and vary the suggested arrangements to best suit his-'particular recording requirements.

In addition, it isto be noted that the time constants of the circuits are of importance and will be adjusted in accordance with the requirements of the particular system; those controlling the limiting action being selected, in most applications, so that no audible signal results from the action of the limitera The interpretation to be applied to the relative terms, low frequencies, medium frequencies and high frequencies, will depend upon the particular frequency range which is being handled by the system. In the conventional sound` recording system in which music is being recorded for entertainment purposes, the frequency ranges may be defined as follows: low frequencies are those below 400 cyclesper second, medium or 'middle range frequencies extend from 400 to 1500 cycles per second, and highfrequencies: are those above 1500 cycles per second. It is obvious that'these a desired volume level and, thus, securing a longer recording time by virtue of the increased number of grooves.

I claim: Y y

1. Apparatus for producing a variable impedance to alternating current and in which the magnitude of said impedance varies in accordance with variations in an applied direct control voltage, said apparatus comprising a transformer having a first and a second Winding, a first and a second vacuum tube each having an anode, first and secondy control electrodes, and an electron emitting element, a circuit connecting said anode of said first tube Vwith one end of said second Winding and said anode of said second tube with the opposite end of said second Winding, a feedback circuit between said anode of said first tube and said first control electrode thereof, a feedback circuit between said anode of said second tube and said first control electrode thereof, andk control electrodes, land an electron emitting element, each of'said tubes'being coupled to said source of voltage', a circuit-connectingsaidanode of said first tube wtih one end of said'second Winding and said anode of said second tube with the opposite end of said second winding, a first and a second negative feed-back circuit coupled respectively to'said secondary electrodes of said first and second tubes, and acircuit connected Yto said primary control electrode of said iirst tube and ysaid primarycontrol electrode` of said second tube for applying a controll voltagewith like polarity to said electrodes, whereby the alternating currentimpedance of said v`first winding is caused to vary in accordance with the magnitude ofrsaid control voltage. Y n Y 3. A sound recording system comprising a record medium, a recorder head, a signal input channel, first, second, and third auxiliary channels,` at least one dividing network coupling said input channelv to each of said auxiliary channels .soY that saidiirst channel carries ,predominantly low frequency components, said second channel carries predominantly middle frequency components, and said third channel carries predominantly highfrequency components, an electrical circuit for re-combining signals from said first, second, and third channels and `feeding them. to said recorder head, rst, second, and third amplifiers included in said first, Second, and third auxiliary channels, respectively, means for varying the gain of said first amplifier in accordance with the variation in amplitude of the low frequency components of said signal,

Yand means for 4varying the gain of said third amplifier in accordance4 with the variation in total voltage applied to said recorder head.

4. A sound recordingv system comprising a record, a recorder head in contact with said record, driving means for rotating said record relative to said recorder head, a signal input channel, first, second, and third auxiliary channels, at least one dividing network coupling said input channel to each of said auxiliary channels so that said first channel carries predominantly loW frequency components, said second channel carries predominantly middle Vfrequency components, and said third channel carries predominantly high frequency components, an electrical circuit for re-combining signals from said first, second, and third channels and feeding them to said recorder head, first, second, and third amplifiers included in said first, second, and third channels, respectively, means for vary- Y.116 ing the gain of said iirst amplifier in: accordance with the variation in amplitude ofnthe low frequency components of said i s'ignaljmeans for modifying the gain of said third amplifier in accordance with the Vvariation in total voltage applied to said lrecorder head,` and means for further modifying the degree of effect of said .total voltage on'the gain of said third amplifier in accordance withv the speed "of said record medium relative to ksaidv recorder head at the point of contact,

' 5. A sound recording system comprising a record, a recorder head, means for rotating said record and moving said recorder head transversely thereof so as to incribe a spiral sound groove thereon, a signal input channel, iirst second, and thirdauxiliary channels, at least one ldividing network coupling said input channel to each of saidauxiliary channels so thatY said first channel carries predominantly 10W frequency components, said second channel carries predominantly ymiddle frequency components, and said third channel carries predominantly high frequency components, an electrical circuit for re-combining signals from said first, secondi and third channels land feeding them to said recorder head, first, second, and third ampliiiers included in said first, second, and third channels, respectively, said iirst and third amplifiers each `hav.

ing a control element for varying theg'ain thereof in accordance With the valueof a D f-C; control signal, means for applying a iirst'D.-C.` control signalV to said first amplifier to vary the gain thereof in `accordance withthe'variationn amplitude of the low* frequency Vcomponents of said signal, a voltage-generating circuit coupled to the output of said re-cornbining circuit for producing a second D n-C. control signal having a magnitude proportional tothe total Vsignal voltage delivered to said recorder head as a result of said re-combined signal, 'a coupling circuit for coupling said second control' signal to said control element of said third amplifier, an

Vadjustable attenuatorV included in said coupling circuit, and a mechanism for adjusting said attenuator in accordanceA with the radial position Vofsaid recorder head relative to saidnrecord. Y'

EMORY G. COOK. REFERENCES CITED The following references are of vrecord inthe file of this patent: 1 UNITED STATES PA'IEN'I'S` Number I NameH., Datel 2,008,7 08y Hammond July 23, 1935 2,096,760 Purington Oct. 276, 19,37 2,122,193 Bedford V;June 28, 1938 2,118,602 Goldsmith May'24, 1938 2,173,222 Belar Sept. v19, 1939 2,239,042 Y Kleber Apr. 22, 1941 2,340,364 Bedford, V 'Feuv 1, 1944 2,369,088 Y Thompson Feb. 6, 1945 2,371,285 Crosby Mar; 1'3, 1945 2,395,159 Albin Feb. 19, 1946 2,528,457 Stone Oct. 31, 1950 

